Techniques as described below are known as to a stripping urea process.
Patent Document 1 has described a urea process using ammonia and carbon dioxide as raw materials, wherein urea synthesis is effected wholly or partly in a combi-reactor. In this process disclosed therein, a gas from a stripper is supplied to the vertical combi-reactor and condensed wholly or partially in ammonium carbamate, which is in turn transferred from a scrubber zone to a condensation section via a downcomer, and ammonia and carbon dioxide are partially synthesized into urea in the condensation zone of the combi-reactor while the further conversion to urea is conducted in the reaction zone of the combi-reactor.
Patent Document 2 has described a combined reactor installation using ammonia and carbon dioxide as raw materials, which is composed of two vertically installed synthesis zones and one condensation zone. This installation is a vertical combined reactor in which two reaction zones are separated by a high-pressure condensation zone. The document has also disclosed another combined reactor including two reaction zones and a high-pressure condensation zone placed outside the reactor, and has further disclosed a process using this installation. Moreover, a method involving wholly or partly feeding a gas supplied from a stripper to the high-pressure condensation zone has also been disclosed therein. In the disclosure, it is preferred that the gas from the stripper is supplied via an ammonia ejector to the second reaction zone in the vertically installed combined reactor.
Patent Document 3 has described an improving method capable of placing an apparatus on the ground in a synthesis method of urea including a stripping step of unreacted ammonia and carbon dioxide at a pressure nearly equal to the pressure of urea synthesis by raw material carbon dioxide and a condensation step of a gas mixture from the stripping step. In this synthesis method of urea disclosed therein, a vertical condenser for bringing a gas mixture from a stripper into contact with an absorbing medium under cooling and thereby condensing the gas mixture is provided above a urea synthesizing tower, and a first down pipe for communicating the top of the condenser to the bottom of the synthesizing tower is provided, whereby the produced condensed liquid is made to flow to the bottom of the synthesizing tower by gravity and then subjected to urea synthesis together with a part of raw material ammonia or carbon dioxide fed thereto, and the produced urea synthesis solution is introduced through a second down pipe having an opening in the top of the synthesizing tower into the stripper by gravity, in which unreacted ammonia and carbon dioxide are then separated as the gas mixture by the remainder of raw material carbon dioxide, then introduced to the bottom of the condenser, and condensed, or alternatively, a condensed liquid from the vertical condenser is sucked by an ejector using preheated raw material ammonia as a driving fluid, then introduced into the bottom of the urea synthesizing tower, and subjected to urea synthesis.
Patent Document 4 has described a method for synthesizing urea with a small volume of necessary equipment per unit production amount, wherein the condensation of a gas mixture of unreacted ammonia and carbon dioxide and the synthesis of urea are performed in a single vessel.
In the disclosure, this method for synthesizing urea includes: feeding a gas mixture obtained by stripping unreacted ammonia and unreacted carbon dioxide with use of raw material carbon dioxide and an absorbing medium to the bottom of a vertical condensation synthesizing tower; feeding raw material liquid ammonia to the bottom and middle of the vertical condensation synthesizing tower; condensing the gas mixture by cooling the part ranging from the bottom to the middle of the vertical condensation synthesizing tower while carrying out the synthesis of urea; introducing the generated urea synthesis solution into the top of a stripper from the top of the vertical condensation synthesizing tower; and subjecting unreacted ammonia and carbon dioxide in the urea synthesis solution to stripping with use of raw material carbon dioxide.
Patent Document 5 has disclosed that the height of an apparatus for a synthesis step can be reduced drastically in a CO2 stripping urea process by placing a synthesis reactor horizontally.
Patent Documents 6 and 7 have disclosed a process wherein a urea synthesis solution coming out of a horizontal submerged condenser is introduced into a synthesis reactor with use of an ejector.    Patent Document 1: International Publication WO 00/43358    Patent Document 2: International Publication WO 01/72700    Patent Document 3: Japanese Patent Laid-Open No. 10-182587    Patent Document 4: Japanese Patent Laid-Open No. 2002-20360    Patent Document 5: Japanese Patent Laid-Open No. 59-122452    Patent Document 6: Japanese Patent Laid-Open No. 11-180942    Patent Document 7: International Publication WO 00/00466